Passover (פּסח) ("Pesach"), also known liturgically as חג המצות ("Ḥag haMatzot", the "Festival of Unleavened Bread"), is one of the Three Pilgrimage Festivals (shalosh regalim) mentioned in the Bible. Passover commemorates the liberation of the Israelite slaves from Egypt. Passover is an eight day festival celebrated from the 15th through 22nd of the hebrew month Nissan(April). To commemorate the unleavened bread that the Israelites ate when they left Egypt the bread didn't rise because they left in such a hurry, we don’t eat any chametz from midday of the day before Passover until the conclusion of the holiday. Chametz means leavened grain—any food or drink that contains even a trace of wheat, barley, rye, oats, spelt or their derivatives, and which wasn’t guarded from leavening or fermentation. This includes bread, cake, cookies, cereal, pasta and most alcoholic beverages. Instead of chametz, we eat matzah—flat unleavened bread. It is a mitzvah to partake of matzah on the two Seder nights. Source